This invention relates to a method of and an apparatus for determining the effective flow area of a restriction in a gas flow passage. It relates in particular, though not exclusively, to a method for measuring the vane (or nozzle) area of a gas turbine engine, and for vane matching of the vane area of a compressor turbine and a power turbine.
The size of a vane (known also as a nozzle) has a significant impact in the performance of gas turbine engines because it changes the operating point on the compressor map. Any change in the turbine vane area rematches the engine to a different gas generator speed, massflow and compressor pressure ratio. For instance, increasing the compressor turbine (CT) vane area while leaving the power turbine (PT) vane area constant has the effect of decreasing the gas generator speed, massflow and compressor pressure ratio at constant output power. Conversely, increasing the PT vane area while leaving the CT vane area unchanged causes the engine to increase in gas generator speed, massflow and compressor pressure ratio at constant output power.
Vane matching based on effective flow area is a crucial engine overhaul procedure for predicting optimum engine performance and achieving optimum efficiency and energy consumption. Improperly matched vanes cause poorer-than-expected engine performance, often resulting in engine failure on test, and increases fuel consumption.
Conventionally, because of the unacceptably high cost and the difficulty of providing a steady sonic rate gas flow, most engine overhaul facilities u se a flow rig that measures the vane area at sub-sonic flows. In actual engine operation, however, the vanes are (or nearly) choked and the gas velocity at the vane's throat is at (or near) the speed of sound. Because of its inability to simulate actual sonic flows the sub-sonic flow rig provides less accurate and less consistent measurement of the vane's area, thus compounding the problem of engine rejects due to incorrect vane match.